1. Field of the Invention
This invention relates to a semiconductor device having an active circuit constituted by thin film transistors formed over a substrate, and a method of fabricating the semiconductor device. More particularly, this invention can be used appropriately for fabricating thin film transistors using a crystalline semiconductor layer. The present invention can be utilized also for fabricating an integrated circuit that comprises thin film transistors, an electro-optical device typified by an active matrix display device, and an image sensor, and an electronic appliance having the electro-optical device mounted thereto.
2. Description of the Related Art
A semiconductor device typified by an active matrix liquid crystal display device comprising a large number of thin film transistors (TFTs) arranged on a substrate has been developed. In order to accomplish high mobility in the TFTs, it has been believed preferable to use a crystalline semiconductor film for a semiconductor layer. Most of polycrystalline semiconductors that are utilized for the TFT are formed by crystallization technologies such as laser annealing and thermal annealing. However, because a large number of defects exist in the crystalline semiconductor film so crystallized, mobility of the carriers and the life time are markedly lowered with the result that electric characteristics of the TFT are adversely affected.
To eliminate the defects inside the crystalline semiconductor film and an inter-layer insulating film, a hydrogenation process has been known as one of the effective means. The hydrogenation process includes a plasma hydrogenation process that neutralizes the defects by generating hydrogen plasma, and a hydrogenation method that executes heat-treatment in a hydrogen atmosphere. These hydrogen process steps are appropriately incorporated in the fabrication process steps of the TFT.
According to the hydrogenation process utilizing the plasma, however, the crystalline semiconductor film is directly exposed to the plasma in order to effectively introduce hydrogen. In consequence, there remains the problem that the crystalline semiconductor film is damaged by the ions that are formed simultaneously in the plasma. To recover this damage, heat-treatment at 400 to 600° C. is believed necessary, but when heating is made to 400° C. or above, hydrogen is re-emitted from the crystalline semiconductor film. If any atmospheric components such as nitrogen and oxygen remain in the atmosphere in the plasma hydrogenation method, these elements, too, are converted to the plasma and contaminate the crystalline semiconductor film.
When the plasma hydrogenation method is carried out from the surface side of an inter-layer insulating film formed on a gate electrode, the defects existing inside the crystalline semiconductor film can be neutralized to a certain extent. However, the hydrogen concentration introduced into the film by this method has a distribution such that it progressively decreases from the surface to its inside. For this reason, it has been difficult to sufficiently hydrogenate the crystalline semiconductor film on the lower layer side. The heat-treatment process in the hydrogen atmosphere as another method involves the problem that the process time gets unavoidably extended in order to improve the hydrogenation effect.